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 A HISTORY OF ESSEX ephippium.' * Subsequently however he found that although this structure in the Chydorus might be regarded as the most highly evolved of the proto-ephippia hitherto recorded, an advance upon it was made in no less than three respects by the homologous structure in Leydigia acanthocercoides, of which he says : ' First, it is formed, as in the Daphnidae, from a much more limited portion of the shell, bounded approximately by a semicircle described upon the dorsal margin ; secondly, it possesses a thick inner coat of specially formed spongy tissue ; and thirdly, it is provided with large hook-like appendages. In spite of these advances in complexity however it does not quite reach the level of the Daphnidan ephippium, because its outer coat is not specially altered beyond the mere deposit of pigment and some extra chitin perhaps, whereas in typical ephippia there is always a more or less abundant development of closely-set hexagonal prismatic cells which are quite independent of the original cell structure, and which, becoming readily filled with air, render the ephippium lighter than water.' Of the Ostracoda all but one in Mr. Scourfield's list belong to the family Cypridids, in which the valves are usually smooth and the hinge not toothed. They are Cypria exsculpta (Fischer) ; C. ophthalmica (Jurine) ; Cyclocypris serena (Koch) ; C. /avis (O. F. Miiller) ; Cypris fuscata, Jurine ; C. incongruens (Ramdohr) ; C. virens, Jurine, a large species which ' is essentially an inhabitant of very small pieces of water, especially such as are well stocked with aquatic vegetation ' ; C. (?) reticu/ata, Zaddach ; ' C. tessellata (in part), Brady [1868] : only the immature " tessellata " forms from Wanstead Park have hitherto been seen, so that it is still a little uncertain whether they should be referred to C. reticulata or not ' ; C. obliqua, Brady ; Erpetocypris reptans (Baird) ; E. strigata (O. F. Miiller) ; E. tumefacta (Brady and Robertson), this and the two preceding species belonging properly to Erpetocypris as instituted in 1889, not to Herpetocypris as proposed in 1896, the name in either form alluding to the creeping habit, as to which Norman and Brady write : ' The power of swimming is lost, and the habits of the animals, which creep along the bottom, are thus very different from those of Cypris ' ; 2 Prionocypris serrata, Norman ; Cypridopsis vi//osa (Jurine) ; Pionocypris vidua (O. F. Miiller) ; Notodromas monacha (O. F. Miiller) ; Ilyocypris gibba (Ramdohr) ; Candona candiaa (O. F. Miiller) ; C. /actea, Baird ; C. compressa (Koch) ; C, pubescent (Koch) ; C.fabceformis (Fischer). In regard to C. pubescens the remark is made that ' within the Epping Forest area it has only been found at Wanstead Park, and this is in fact its only known British locality, as the reference to Pavenham [Bedford- shire] in Brady and Norman's Monograph [part z] p. 729 was made under a misunderstanding.' The name of the genus Notodromas, Lillje- borg, signifies a dorsal runner. Of animals that can move easily with the back downwards house-flies on our ceilings are familiar examples. Many kinds of Entomostraca swim by preference on their backs. In 1 The Annual of Microicopy, p. 64 (Oct. 1898). 216
 * Trans. Royal Dublin Soc. ser. 2, vol. iv. p. 84 (1889).